The present invention relates to a system for measuring the luminance characteristics of objects, particularly objects with luminance dependent on the emission direction.
It applies for example to projection screens, to cathode ray tubes, to lighting devices and to display screens such as liquid crystal displays, plasma displays, electroluminescent displays and microtip screens.
Several systems for measuring the luminance characteristics of objects in accordance with a geometrical position so as to characterise their uniformity and properties are already known. Such systems may be used with the objects given above for example.
In particular an electromechanical system is known which is diagrammatically shown in FIG. 1.
This system includes:
a measuring instrument 2, for example a photometer, and
movement means 4 of this instrument in front of an object to be measured 6 (these means 4 being symbolised by arrows in FIG. 1).
This electromechanical system allows measurements to be taken along the optical axis X of the measuring instrument (and therefore according to an angle xcex8 zero relative to this axis) but has however numerous drawbacks. In particular the measurements are taken by sampling. Only the positions selected are measured and no information is known about luminance in the intermediate positions. No certainty exists as to the value of the luminance outside that of the points measured. Moreover, the measurements, of duration T0, are taken in series, one after the other. If a great number N of points are to be measured in order to be able to have maximum information, the complete measurement of the object takes a time NXT0.
Another measurement system is also known and this is diagrammatically shown in FIG. 2.
This system includes:
a matrix sensor 8 of the CCD type or similar and
a lens 10 which is between this sensor and the object 6 to be measured and allows the image of this latter to be formed on the sensor.
Thus, at one and the same time, an image of the object to be measured is obtained on the sensor. The different points of the image correspond to the measurements relative to the different points of the object to be measured.
The main advantages of this sensor system are as follows:
Measurement speed is increased. Indeed, measurement, of duration T1, does not depend (or only slightly) on the number of points measured. All the information is available. There is no risk of seeing a detail of the image evade measurement. An integration (summation) of all the values obtained gives with certainty a value of the luminous flux emitted by the object.
However the system shown in FIG. 2 has a serious drawback which is shown diagrammatically by FIG. 3. The lens 10, the axis of which is denoted X and which is conventionally used for such a system, operates at a constant image size D2=2d2. An object of size D2=2d1 must therefore be at a distance L1 from this lens such that:
L1/(2d1)=L2/(2d2)=K
where L2 is the distance between the lens 10 and the sensor 8 and K is a constant.
The object is observed along an angle xcex8 which depends on the measured point of the object (xcex8 is counted relative to a straight line passing through this point and parallel to the optical axis X of the lens 10) and which, for the end points, takes a value xcex8M (FIG. 2) little different from tan xcex8M and therefore little different from d2/L2 in other words from 1/(2K).
Usually K is of the order of 2-5 to 3 (which means it is necessary to place 75 cm from the lens a 12xe2x80x2 (about 30 cm) diagonal screen which it is wished to measure) so that xcex8M is of the order of 12xc2x0. Measurements are therefore taken at a variable angle, according to the position, between 0xc2x0 (measurement along the axis X) and xc2x112xc2x0.
This would not be a drawback if the objects measured had an emission characteristic such that the luminance does not vary in accordance with the light emission direction in other words in accordance with the angle xcex8.
This is not usually the case and it is clear that a system of the type of that in FIGS. 2 and 3 does not allow the emission uniformity of an object to be measured independently of the emission characteristic of that object.
The purpose of the present invention is to overcome this drawback.
It relates a system allowing the luminance characteristics of objects to be accurately measured, whether or not their luminance varies in accordance with the emission direction.
The invention combines the advantages of the system in FIG. 1 (with which measurements are taken at xcex8=0xc2x0) and those of the system in FIG. 2 (with which the measurements are taken rapidly).
In an exact way, an object of the present invention is a system for measuring the luminance characteristics of objects, this system including:
an image sensor and
optical means having an optical axis and provided to form the image of the totality of an object on the sensor, each point of the image allowing a measurement to be taken on a point of the object,
this system being characterised in that the optical means are additionally provided to select, for each point of the object, with a view to forming the corresponding image-point, those of the light rays coming from this point of the object which propagate in a way approximately parallel to the optical axis of the optical means.
According to a preferred embodiment of the system object of the invention, the optical means include:
a first lens placed facing the object,
a diaphragm placed between the first lens and the sensor and able to let pass, among the light rays coming to it from the object through the first lens, only those which propagate from the object in a way approximately parallel to the optical axis of the first lens, and
auxiliary optical means placed between the diaphragm and the sensor and provided to form, from the light rays which the diaphragm lets pass, the image of the object in an observation plane, the sensor being approximately placed in this observation plane.
Preferably, the object is approximately placed in the object focal plane of the first lens and the diaphragm is approximately placed in the image focal plane of this first lens.
According to a first particular embodiment of the invention, the auxiliary optical means include a second lens provided to form the image of the object in the observation plane.
According to a second particular embodiment, the auxiliary optical means include:
a second lens provided to form an intermediate image of the object in an intermediate plane and
a third lens placed between the second lens and the sensor and provided to form the image of the object in the observation plane from the intermediate image and to adapt the size of the image of the object to the size of the sensor.
The aperture of the diaphragm can be variable. Moreover, the system object of the invention can include an optical filtering means of the light coming from the object.
In the case of the first particular embodiment mentioned above, this filtering means is preferably approximately placed in the observation plane, facing the sensor. In the case of the second particular embodiment, this filtering means is preferably approximately placed in the intermediate plane.
In the case of one or the other of these particular embodiments, the second lens is preferably provided in order that the light rays which come from the object and which reach the optical filtering means are perpendicular to the plane where this optical filtering means is located.
The sensor is preferably of the matrix type.
Indeed a device for surveying the luminous emission properties of a light emitting surface is known from the document FR2715470A. However, in this document, it is a question of measuring average intensity on a surface delimited by a diaphragm, by means of a single sensor. The information obtained is a single quantity. The means implemented aim to obtain a uniform response over a controlled surface size.
On the contrary, in the present invention, it is a question of measuring the luminance, point by point, of an extended object. The information obtained is a set of quantities (cartography). The means implemented aim to make an observation at a constant angle.